Soaring television costs accounted for more than half the spending in the presidential campaign of 1992, a greater proportion than it was in any previous election.
比较的对象是Soaring television costs accounted for more than half the spending 简单的说就是proportion, 并不是television costs。是拿costs占spending的比例来做比较。所以我不认为c里面应该用they。
其次我觉得B不对是因为比较大双方一个是1992年的proportion另一个是in any previous election。proportion和eleciton不能比较。A的话就是porportion和it was in any previous election比较,是porportion比较porortion。